1. Field of the Invention
The present invention relates generally to a fluid sampling tool which uses well pressure in an oil or gas well to move a valve to allow a well fluid sample to be taken. More particularly, but not by way of limitation, the invention relates to various means for controlling the operation of such a sampling tool.
2. Description of the Prior Art
In general, to obtain a sample, a fluid sampling tool is first lowered into the well on a tubing string or a wire line or a slick line. When the tool is at the desired depth, a port (one or more openings) defined in the tool is opened. The port may open in response to pressure exerted through the well fluid or in response to an electrical actuation signal from the surface. The open port admits well fluid into a sample retaining chamber within the tool. The port is thereafter closed, the tool is withdrawn from the well, and the sample is taken from the chamber for analysis.
Zunkel U.S. Pat. No. 4,903,765, and assigned to the assignee of the present invention, shows a recent improvement in such fluid sampling tools, wherein the fluid sampling tool is constructed to have a time delay which starts when a valve of the tool first starts to move in response to pressure from the well. This time delay provides various advantages. In one instance, the time delay allows undesired fluids such as drilling fluids to bypass the sampling tool before the valve communicates a sample port with a sample chamber and a sample of the well fluid is taken. In another instance, the time delay can reduce the dependency on accurate pressure readings and shear pins which control the opening of the valve. For example, when a maximum bottom hole pressure is measured or otherwise anticipated, shear pins providing a holding force of something less than this maximum pressure, but one which will clearly be encountered somewhere downhole despite a lack of assurance as to precisely where it will be, can be used so that the pins will break at some location above the bottom of the well. The time delay, designed with a suitable tolerance to assure reaching bottom before its expiration, is then used to allow the tool to be run on down to the well bottom, where it will ultimately automatically open.
The present invention provides various improvements upon a delayed opening fluid sampler of the type generally shown in Zunkel U.S. Pat. No. 4,903,765. The improvements relate generally to various means for controlling the actuation of the valve which controls flow of the sample fluid to the sample chamber.